1. Field of the Invention
The present invention relates to a piezoelectric actuator, utilizing a reverse piezoelectric effect and an electrostrictive effect under the influence of a large electric field, such as; a laminated actuator, a piezoelectric transformer, an ultrasonic motor, a bimorph piezoelectric element, an ultrasonic sonar, a piezoelectric ultrasonic vibration element, a piezoelectric buzzer, or a piezoelectric loudspeaker.
2. Description of the Related Art
Piezoelectric actuators employing piezoelectric ceramic materials are industrial products that convert electric energy into mechanical energy by utilizing displacement derived from a reverse piezoelectric effect, and are widely applied to the fields of electronics and mechatronics.
As the piezoelectric ceramics adapted to the piezoelectric actuators, for example, a lead zirconium titanate (Pb(Zr,Ti)O3) system (hereinafter a PZT system) and barium titanate (BaTiO3) are known. The PZT-system piezoelectric ceramics exhibit higher piezoelectric properties than the other piezoelectric ceramics, and are used for the majority of piezoelectric ceramics that are currently applied to practice use. However, as the PZT-system piezoelectric ceramics contain lead oxide exhibiting a high vapor pressure, a load they impose on an environment is large. On the other hand, the BaTiO3 ceramic does not contain lead, but exhibits lower piezoelectric properties than PZT. Moreover, as the Curie temperature of the BaTiO3 ceramic is as low as approximately 120° C., the BaTiO3 ceramic cannot be used at high temperatures.
The piezoelectric actuator generally includes: a piezoelectric element, which has at least one pair of electrodes formed on a piezoelectric ceramic member; a holding part that holds the piezoelectric element; an adhesive member or a press-contacting member such as a spring that retains the piezoelectric element in the holding part; a lead terminal via which a voltage is applied to the piezoelectric element; an electrical insulation member such as a resin or silicone oil that is coated over the pair of electrodes. In the piezoelectric actuator, the piezoelectric element including the piezoelectric ceramic member is retained by means of adhesion, molding, or a spring. Therefore, a mechanical restraining force (pre-set load) is applied, while no voltage is applied. Moreover, in the piezoelectric actuator, when a voltage is applied to the piezoelectric actuator, the piezoelectric element is displaced along with a rise in the voltage. This intensifies the mechanically restraining force (increases the load).
Consequently, the displacement of the piezoelectric actuator is, unlike the displacement performance of the piezoelectric element itself, smaller due to a presetting load and an increase in the load.
The working conditions and driving conditions for the piezoelectric actuator include such parameters as the temperature, driving electric-field strength, driving waveform, driving frequency, and whether a driving mode is continuous driving or intermittent driving. As for a general working temperature range for the piezoelectric actuator, when the piezoelectric actuator is used in a general living environment, the largest working temperature range is from about −30° C. to about 80° C. When the piezoelectric actuator is adopted as an automotive part, the largest working temperature range is from about −40° C. to about 160° C. Moreover, the amplitude in electric field strength of a driving voltage varies depending on the usage of the piezoelectric actuator. For a piezoelectric buzzer, an ultrasonic sonar, a piezoelectric loudspeaker, or the like, the amplitude is equal to or smaller than 500 V/mm. For an ultrasonic motor, a piezoelectric transformer, a piezoelectric ultrasonic vibration element, or the like, the amplitude is equal to or smaller than 1000 V/mm. For a laminated actuator, the amplitude is equal to or smaller than 3000 V/mm. Moreover, when resonant driving is adopted as a driving form, the driving waveform is a sine wave. For the other driving forms, the driving waveform may be any of various waves, that is, the sine wave, a trapezoidal wave, a triangular wave, a rectangular wave, and a pulsating wave. Moreover, the driving frequency employed in the ultrasonic motor, ultrasonic sonar, piezoelectric ultrasonic vibration element, or the like is equal to or higher than 20 kHz, while the driving frequency employed in the other products falls below 20 kHz.
The driving methods for the piezoelectric actuator are classified into (1) a constant-voltage driving method for driving the piezoelectric actuator by controlling a displacement using a voltage as a parameter, (2) a constant-energy driving method for driving the piezoelectric actuator by controlling a displacement using injected energy as a parameter, and (3) a constant-charge driving method for driving the piezoelectric actuator by controlling a displacement using injected charge as a parameter.
Now, the relationship between each of the driving methods and a displacement made by the actuator will be described below.
A piezoelectric actuator driving method classified into the constant-voltage driving method is characterized in that a displacement occurring during the rising of an applied voltage and a displacement occurring during the falling thereof have a hysteresis. The constant-voltage driving method has a drawback that the variation width in a displacement over a working temperature range is relatively large.
Moreover, a piezoelectric actuator driving method classified into the constant-energy driving method is characterized in that a displacement occurring during an increase of injected energy and a displacement occurring during a decrease thereof have a hysteresis. In the constant-energy driving method, the variation width in a displacement over the working temperature range is smaller than that in the constant-voltage driving method.
On the other hand, an actuator driving method classified into the constant-charge driving method is superior in a point that a displacement can be most precisely controlled, because the difference between a displacement occurring during an increase of injected charge and a displacement occurring during a decrease thereof is nearly zero. However, the variation width in a displacement over the working temperature range is larger than those in the constant-voltage driving method and the constant-energy driving method respectively.
As a method for diminishing the variation width in a temperature characteristic of a piezoelectric actuator or a piezoelectric ceramic sensor, technologies described below have been developed.
Specifically, Japanese Unexamined Patent Publication No. 60-1877 has disclosed a piezoelectric body having a piezoelectric unit, of which displacement that is an output provided in response to application of a voltage changes as an increasing function of a change in temperature, and a piezoelectric unit, of which displacement changes as a decreasing function thereof, combined and stacked up.
Moreover, Japanese Unexamined Patent Publication No. 6-232465 has disclosed a laminated piezoelectric actuator having multiple piezoelectric ceramic layers, which are different from one another in terms of displacement performance, stacked up.
Japanese Unexamined Patent Publication No. 5-284600 has disclosed a piezoelectric element having a temperature compensation capacitor electrically connected in series or parallel with a piezoelectric ceramic member.
Japanese Unexamined Patent Publication No. 7-79022 has disclosed a piezoelectric element that generates charge according to a pressure. In the piezoelectric element, a piezoelectric substance layer and a dielectric layer are alternately stacked. The electrostatic capacity of the dielectric layer is larger than the electrostatic capacity of the piezoelectric layer. Moreover, the temperature coefficient of the dielectric layer is the reverse of the temperature coefficient of the piezoelectric layer.
Japanese Unexamined Patent Publication No. 7-79023 has disclosed a piezoelectric element that generates charge according to a pressure. Herein, a piezoelectric substance material and a dielectric body material, whose electrostatic capacity varies with temperature characteristic reverse to that of the piezoelectric substance material, are mixed in order to mold the piezoelectric element.
Moreover, Japanese Unexamined Patent Publication No. 11-180766 has disclosed a barium titanate-system piezoelectric porcelain that is a composition whose piezoelectric d33 constant measured according to a resonance method is equal to or larger than 300 pC/N. The piezoelectric d33 constant exhibits a small temperature-dependent change rate at the temperatures ranging from −30° C. to 85° C.
Japanese Unexamined Patent Publication No. 2003-128460 has disclosed a laminated piezoelectric element in which an internal electrode is made of nickel (Ni) that is a barium titanate-system. Herein, the temperature-dependent change rate of a piezoelectric d31 constant calculated from a distortion factor of the element applied the electric field strength of 1 kV/mm is small.
However, the conventional technologies have failed to resolve a fluctuation in the displacement characteristic of a piezoelectric actuator derived from a change in temperature.